Effect of Altitude on Gasoline Combustion Efficiency in Light-Duty Vehicles: Evidence from Andean Corridors (0–4000 m a.s.l.)

Reduced air density at altitude challenges spark-ignition combustion in light-duty vehicles, particularly in Andean countries where fleets operate between sea level and over 4000 m a.s.l. We quantified the effect of altitude on gasoline combustion efficiency and pollutant formation using 94,263 second-by-second records (2021–2025) from ten Euro 2–5 vehicles travelling Andean and coastal corridors in Ecuador (0–4000 m a.s.l.). Emissions were measured with a Brain Bee AGS-688 analyser with pressure compensation; fuel consumption via OBD-II; operational demand via smoothed Vehicle Specific Power (PSVm10) in six K-Means clusters. We propose RCO=[CO]/[CO2] and RHC=[HC]/[CO2] as standardised, displacement-independent combustion quality indicators. RCO at 3500–4000 m exceeded the 500–1000 m value by 7.5×, and RHC by 18×. Nitric oxide showed a non-linear “N”-pattern, peaking locally at 2000–2500 m (212 ppm) and absolutely at 3500–4000 m (613 ppm), linked to EGR suppression and rising chamber temperature. CO emission factors reached 6.42 g/km at 2000–2500 m versus 1.32 g/km at the coast (4.9×). These results provide the first naturalistic evidence base for calibrating high-altitude emission inventories in Andean corridors.